A buck-boost DC-DC converter refers to a power converter in which an input operating voltage delivered to the converter may span a range of values extending below and above the magnitude of the DC voltage delivered at the output of the converter. Buck-boost DC-DC converters find various applications in electrical and electronic equipment and systems. As an example, in a stationary or portable system powered by a DC battery, it may be desirable to maintain the output voltage at a substantially constant level regardless of the state of charge and voltage of the battery.
In a non-idealized switching converter, i.e., switches used in the converter have parasitic capacitances and inductances, power may be dissipated in a switch when the switch is being turned on, which is called “switching loss.” Capacitances, both parasitic and lumped, across a switch if not discharged before the switch is turned ON may be a major contributor to switching loss. One way to reduce switching loss in a switching power converter (e.g., a buck, a boost, or a buck-boost switching power converter) uses an inductive current to fully or partially charge and discharge the capacitances associated with a switch before turning it ON to achieve full or partial zero voltage switching (“ZVS”) during an energy recycling interval (“ERI”) (which may also be called a “ZVS” interval). ZVS ideally causes the voltage across the switch to decline to zero volts, essentially eliminating switching losses associated with the capacitive discharge of the switch; however, any significant reduction, e.g. by 50 percent, 80 percent, 90 percent, or more from the peak voltage across the switch, respectively reduces the switching losses during turn ON by 75 percent, 96 percent, 99 percent, or more. However, it may be difficult to control switches to turn ON or OFF at times when the voltage across the switch is at zero volts or a minimum voltage due to a variety of factors, including fast voltage or current transitions, very small signals, propagation delays, and noise in the converter. Turning switches ON and OFF at times when zero or minimal current is flowing through the switch, called zero current switching (“ZCS”), can also reduce losses and reduce noise. However, ZCS operation of switches also may be difficult for the same reasons as with ZVS.
Besides difficulty in ZVS and ZCS operations, it may also be difficult to sense the output current for a converter without dissipating excessive power. Typically, a sense resistor is connected in series with the load at the output side of the converter and the voltage across the sense resistor is sensed to monitor the output current. A trade-off between the need for sufficient signal voltage across the resistor, e.g. relative to noise, and power dissipation in the resistor, may lead to significant power consumption in the resistor, impacting converter efficiency.